Eine neue Dimension in der Proteinkristallographie

Time-resolved crystallography on H-ras p21

We describe here the results obtained to date on a project aimed at characterizing the changes occurring in the protein product (p21) of the H-ras proto-oncogene during and as a result of hydrolysis of GTP at its active site. The approach used involves crystallization of p21 with a photosensitive precursor of GTP (caged GTP) at the active site followed by generation of GTP by photolysis and collection of X-ray diffraction data using the Laue method at a synchrotron source. The structure of p21 complexed with a single diastereomer of caged GTP is presented here. In contrast to crystals obtained with mixed diastereomers, the nucleotide appears to bind in a manner which is very similar to that of other guanine nucleotides (GDP, GTP, GppNHp). The current state of time resolved structural experiments using these crystals is presented.

Studies on the structure and mechanism of H-ras p21

Current knowledge of the structure of H-ras p21 is reviewed with particular emphasis on the interaction between guanine nucleotides and the active site of the protein. The nature of the conformational change induced by GTP hydrolysis is discussed. The major change is seen in the region known as the effector loop (loop 2), with significant but less well-defined changes occurring in loop 4, which is implicated in the GTPase reaction. Other evidence concerning the mechanism of GTP hydrolysis and its activation by GAP (GTPase-activating protein) is also discussed. Evidence regarding the rate limiting step in the p21 GTPase reaction, and the manner in which this and possibly other steps are accelerated by GAP, is inconclusive.